<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<h3>结果:<span></span></h3>

<script>
//的页面中的元素对象
let i1 = document.querySelector("#i1");
let i2 = document.querySelector("#i2");
let result_span = document.querySelector("span");

function f(x) {
    //NaN: Not a Number 代表不是数, NaN和任何数值进行任何运算结果都是NaN
    if (isNaN(i1.value)||isNaN(i2.value)){
        result_span.innerText = "输入错误!";
        return;
    }
   switch (x) {
        case 1:
            result_span.innerText = i1.value*1+i2.value*1;
            break;
       case 2:
           result_span.innerText = i1.value-i2.value;
           break;
       case 3:
           result_span.innerText = i1.value*i2.value;
           break;
       case 4:
           result_span.innerText = i1.value/i2.value;
           break;
   }
}


</script>
</body>
</html>